Gain variable amplifier

Abstract

By a gain control circuit which has: a first differential input circuit comprising a differential pair of transistors Tr 1 and Tr 3 in which transistors Tr 2 and Tr 4 connected to diodes are inserted; a second differential input circuit comprising a pair of the transistor Tr 3 and a transistor Tr 5 ; and a current source circuit comprising a transistor Tr 6 and an emitter resistor R 3 , and to which first to third individual control input signals VCLS 1 to VCLS 3 each having a predetermined inclination and an offset value, obtained by shifting the level of a control input signal VCNT are supplied, gain control signals VAGC having a linear characteristic, a monotone increase characteristic of which curve opens upwards, and a monotone increase characteristic of which curve opens sharply upwards are supplied in accordance with a monotone increase region of which curve opens upwards, a linear region, and a monotone increase region of which curve opens downwards, respectively, of a differential pair of transistors Tr 11 and Tr 12 and a differential pair of transistors Tr 13 and Tr 14 at the second stage of a differential amplification circuit.

What is claimed is:  
     
       1. A gain variable amplifier comprising: 
       a gain control circuit having a gain control characteristic of outputting a predetermined gain control signal in accordance with a control input signal; and  
       a differential amplification circuit having an input output characteristic of varying a gain by switching a differential output current passing through a differential pair by the gain control signal,  
       wherein when the gain control signal is supplied to the differential amplification circuit and is in a first region in which the input output characteristic is a linear characteristic in a linear coordinate system, the gain control circuit variably controls the gain control characteristic so as to be a first monotone increase control characteristic of which curve opens upwards in a linear coordinate system,  
       when the gain control signal is supplied to the differential amplification circuit and is in a second region in which the input output characteristic is a monotonous increase characteristic of which curve opens downwards in a linear coordinate system, the gain control circuit variably controls the gain control characteristic to a second monotone increase control characteristic of which curve is sharper than that of the first monotone increase control characteristic, and  
       when the gain control signal is supplied to the differential amplification circuit and is in a third region in which the input output characteristic is a monotone increase characteristic of which curve opens upwards in a linear coordinate system, the gain control circuit variably controls the gain control characteristic to be a linear control characteristic.  
     
     
       2. A gain variable amplifier according to  claim 1 , wherein the gain control circuit has a transconductance amplifier construction, and variably controls the gain control characteristic by varying a conductance characteristic of the transconductance amplifier construction. 
     
     
       3. A gain variable amplifier according to  claim 2 , wherein the transconductance amplifier construction has a composite differential input circuit of multiple inputs, and 
       an individual control input signal having a predetermined inclination and a predetermined offset value is supplied in response to each input signal supplied to the composite differential input circuit.  
     
     
       4. A gain variable amplifier according to  claim 2 , wherein the gain control circuit comprises: 
       a first transistor and a second transistor constructing a differential pair;  
       a first diode whose anode is connected to an emitter of the first transistor;  
       a second diode whose anode is connected to an emitter of the second transistor;  
       a third transistor;  
       a first differential input circuit in which a cathode of the first diode and a cathode of the second diode are connected to each other, a reference voltage is applied to a base of the first transistor, and a first individual control input signal is supplied to a base of the second transistor;  
       a second differential input circuit in which an emitter of the third transistor is connected to the cathodes of the first and second diodes, the cathodes being connected to each other, a second individual control input signal is supplied to a base of the third transistor, the circuit being achieved with the first transistor;  
       the composite differential input circuit being constructed by the first and second differential input circuits;  
       a fourth transistor; and  
       a resistor connected to an emitter of the fourth transistor,  
       wherein a current source circuit of a predetermined conductance is formed by connecting the other end of the resistor to a reference potential and supplying a third individual control input signal to a base of the fourth transistor, and  
       a sum of an output current of the first differential input circuit, an output current of the second differential input circuit and an output current of the current source circuit is converted to a voltage and the voltage is outputted as the gain control signal.  
     
     
       5. A gain variable amplifier according to  claim 2 , wherein the gain control circuit comprises: 
       a first transistor and a second transistor which form a differential pair;  
       a first diode whose anode is connected to a source of the first transistor;  
       a second diode whose anode is connected to a source of the second transistor;  
       a third transistor;  
       a first differential input circuit in which a cathode of the first diode and a cathode of the second diode are connected to each other, a reference voltage is applied to a gate of the first transistor, and a first individual control input signal is supplied to a gate of the second transistor;  
       a second differential input circuit in which a source of the third transistor is connected to the cathodes of the first and second diodes, the cathodes being connected to each other, a second individual control input signal is supplied to a gate of the third transistor, the circuit being achieved with the first transistor; and  
       the composite differential input circuit being constructed by the first and second differential input circuits,  
       wherein a sum of an output current of the first differential input circuit and an output current of the second differential input circuit is converted to a voltage and the voltage is outputted as the gain control signal.  
     
     
       6. A gain variable amplifier according to  claim 3 , further comprising a level shifting circuit, 
       wherein the individual control input signal having a predetermined inclination and a predetermined offset value is generated by shifting the level of the control input signal.  
     
     
       7. A gain variable amplifier comprising: 
       a gain control circuit inputting a plurality of individual input signals in accordance with a control input signal; and  
       a differential amplification circuit having an input output characteristic of varying a gain by switching a differential output current passing through a differential pair by the gain control signal;  
       wherein the gain control circuit comprises at least three circuits as below:  
       a first circuit inputting a first individual input signal, and outputting a first gain control signal which variably controls the gain control characteristic so as to be a first monotone increase control characteristics of which curve opens upwards in a linear coordinate system when the first gain control signal is supplied to the differential amplification circuit and is in a first region in which the input output characteristic is a linear characteristic in a linear coordinate system;  
       a second circuit inputting a second individual input signal, and outputting a second gain control signal which variably controls the gain control characteristic to a second monotone increase control characteristic of which curve is sharper than that of the first monotone increase control characteristic when the second gain control signal is supplied to the differential amplification circuit and is in a second region in which the input output characteristic is a monotonous increase characteristic of which curve opens downwards in a linear coordinate system; and  
       a third circuit inputting a third individual input signal, and outputting a third gain control signal which variably controls the gain control characteristic to be a linear control characteristic when the third gain control signal is supplied to the differential amplification circuit and is in a third region in which the input output characteristic is a monotone increase characteristic of which curve opens upwards in a linear coordinate system.